Due to a rapid spread of wireless devices represented by mobile phones, demands for duplexers have been increasing rapidly. Particularly, there are strong demands for duplexers that use compact acoustic wave elements having high steepness.
The duplexers include a transmission filter and a reception filter. The transmission filter and the reception filter are required to have characteristics of low loss, high suppression and wide bandwidth. Especially, increasing the bandwidth of the filters has been strongly desired along with the recent demands for high-speed, large-capacity communication.
As a method for realizing the filters using acoustic wave elements, ladder filters have been used widely. The ladder filter is a high-frequency filter configured by connecting a plurality of resonators having different resonance frequencies in a ladder shape.
For increasing the bandwidth of the ladder filter, it is in principle possible to shift the resonance frequencies of the resonators to the lower frequency side by connecting inductances to the resonators. Thereby, an apparent frequency difference between the resonance frequency and antiresonance frequency can be expanded, which allows the filter to have a wider bandwidth. Patent Document 1 discloses a configuration in which the difference between the resonance frequency and the antiresonance frequency is expanded by connecting an inductance to each of series resonators and parallel resonators in series, so that wider passband characteristics can be obtained without causing degradation in the middle band.
Patent Document 1: JP 10 (1998)-126212 A
However, the inventors realized that in the configuration disclosed in Patent Document 1, since inductances are connected to respective resonators, many inductances need be provided along with an increase in the number of filter stages, which prevents a filter chip from being compact. The inventors thought that by connecting inductances only to the parallel resonators, the chip can be made compact. However, in this case, an increase in the bandwidth is limited by the difference between the resonance frequency and the antiresonance frequency of the series resonator, which makes it difficult to sufficiently increase the bandwidth.